The present invention relates to an artificial leather which looks like nubuck, and a production process thereof.
In more detail, the present invention relates to an artificial leather having a less oriented surface nap and having short densely packed nap fibers, hence having a nubuck-like look and hand, and also relates to a production process thereof.
In recent years, techniques for producing artificial leathers having nap produced from synthetic ultra-fine fiber-entangled substrates and elastic polymers have made rapid progress, and those artificial leathers are being widely accepted in the fields of high quality fashion, car sheets, interior, furniture, etc.
These artificial leather production techniques can be said to have shown remarkable progress especially in the area of suede-like artificial leathers, etc. Since these artificial leathers have many ultra-fine fibers raised on the surfaces, the following problem arises at the time of sewing, etc.
Since the nap of a finally produced fabric is highly oriented, a product obtained by sewing plural fabric pieces different in nap orientation visually shows a large color shade difference on the differently oriented nap surfaces. So, to prevent such a difference, it is necessary to sew only the fabric pieces with the same nap orientation together. For this reason, the fabric pieces cannot be effectively used, and there is a limit in the sewing yield.
The visually large color shade difference depending on the observation direction is more likely to be accepted in the artificial leather area in favor of longer nap since it provides a feeling of high quality, but on the other hand, in the artificial leather area in favor of shorter and denser nap like nubuck, such a large color shade difference is less likely to be accepted.
Because of the above problem, it is demanded that an artificial leather with a nap surface small in visual color shade difference depending on the observation direction and good in sewing yield is developed.
Such an artificial leather with small visual color shade difference in the nap surface depending on the observation direction is essentially more suitable for the nubuck-like artificial leather area rather than for the suede-like artificial leather area, if the nap is shorter and denser.
In the conventional production of artificial leathers consisting of ultra-fine fibers and elastic polymers, several techniques for shortening and densifying the nap raised on the surface are known.
For example, Japanese Patent Laid-Open (Kokai) No. Hei7-126986 describes a method comprising the steps of slitting a sheet composed of ultra-fine fibers and an elastic polymer, coating the slit surface with a solution containing a solvent of the elastic polymer, and buffing the surface coated with the solvent-containing solution.
Furthermore, Japanese Patent Laid-Open (Kokai) No. Hei7-126985 proposes a method comprising the steps of impregnating a conjugate fiber sheet with an elastic polymer, partially squeezing the elastic polymer in the normal direction from the surface of the substrate, coagulating, making the conjugate fibers ultra-fine, applying a solvent of the elastic polymer to the non-raised surface, coagulating, and buffering the solvent-applied surface.
The former method has a problem that it is difficult to control the degree of solution impregnation in the normal direction of the sheet, and the latter method has a problem that it is difficult to control the squeezed amount when the sheet thickness fluctuates due to various reasons. As a result, though the denseness of the nap improves, the nap length is likely to be uneven, depending on the degree of buffing, and furthermore, though the nap length can be shortened since the adhesive strength between the ultra-fine fibers and the elastic polymer increases, there is another problem that the product looks less soft on the other hand.
A first object of the present invention is to provide an artificial leather with a nap surface small in visual color shade difference and good in sewing yield. With the successful development of the artificial leather, an unprecedentedly visually excellent nubuck-like artificial leather is intended to be provided.
A second object of the present invention is to provide a process for allowing the production of the above artificial leather, particularly the unprecedentedly visually excellent nubuck-like artificial leather.
A third object of the present invention is to provide a process for allowing the production of the above artificial leather, particularly a nubuck-like artificial leather having a soft hand and having a nap surface very small in visual color shade difference.
The nubuck-like artificial leather of the present invention to achieve the first object of the present invention is as follows.
An artificial leather having a nap surface of ultra-fine fibers at least on one side, which is formed by applying an elastic polymer to an ultra-fine fiber-entangled substrate, characterized by being 0.3 g/cm3 or more in the apparent density of the artificial leather, being 0.5 mm or less in the nap length, and being 25% or less in the value obtained from the following formula based on the goniometric reflectance distribution measured with the nap surface rotated from 0 degree to 180 degrees using a goniophotometer.
R value (%)=(R1xe2x88x92R3)/(R1xe2x88x92R2)xc3x97100 
(where R1 is the quantity of reflected light at 0 degree; R2 is the minimum quantity of reflected light in a range from 0 to 180 degrees; and R3 is the quantity of reflected light at 180 degrees).
The process for producing a nubuck-like artificial leather of the present invention to achieve the second object of the present invention is as follows:
A process for producing a nubuck-like artificial leather, in which a sheet obtained by applying an elastic polymer to an ultra-fine fiber-entangled substrate is raised to produce a napped sheet, comprising the steps of applying an elastic polymer to an ultra-fine fiber-entangled substrate, substantially solidifying the elastic polymer, immersing the polymer-deposited fiber-entangled substrate into a swelling agent of the elastic polymer to swell the elastic polymer, compressing the sheet in the normal direction of the sheet, removing the swelling agent by an aqueous solvent, and raising the sheet at least on one side.
The process for producing a nubuck-like artificial leather to achieve the third object of the present invention is according to the above process, wherein the constraint at the nap base is eased before or after nap raising, for making the nap less oriented, by caustic reducing the ultra-fine fibers or rubbing the nap raised fabric as a whole.
The above artificial leather and the production process thereof of the present invention can provide an artificial leather with a small visual color shade difference in the nap surface and good in the sewing yield, hence an unprecedentedly visually excellent nubuck-like artificial leather, and also a production process thereof.